The role of silicon in drug discovery: a review

This review aims to highlight the role of silicon in drug discovery. Silicon and carbon are often regarded as being similar with silicon located directly beneath carbon in the same group in the periodic table. That being noted, in many instances a clear dichotomy also exists between silicon and carbon, and these differences often lead to vastly different physiochemical and biological properties. As a result, the utility of silicon in drug discovery has attracted significant attention and has grown rapidly over the past decade. This review showcases some recent advances in synthetic organosilicon chemistry and examples of the ways in which silicon has been employed in the drug-discovery field.

Hydroboration of vinylsilanes providing diversity-oriented hydrophobic building blocks for biofunctional molecules

Hydroboration of vinylsilanes with BH3 affords two silylethanol regioisomers. Herein, we investigated the regioisomeric ratio of hydroboration products from various vinylsilanes, focusing on the characteristic reaction profile. All investigated vinylsilanes afforded both regioisomers, and greater bulkiness increased the proportion of the Markovnikov products. The obtained silylethanols were used as hydrophobic building blocks for constructing nuclear progesterone receptor (PR) modulators. Notably, structural conversions from an α-isomer (silylethan-1-oxy derivative) to a β-isomer (2-silylethoxy derivative) caused complete activity-switching from a PR agonist to an antagonist. Our results indicate that silylethanols are useful for structural development, and vinylsilanes are a versatile source of hydrophobic building blocks for obtaining biofunctional molecules.

Magnetic properties and magnetocaloric effect of Ln = Dy, Tb carborane-based metal-organic frameworks

We present the synthesis and magneto-thermal properties of carborane-based lanthanide metal-organic frameworks (MOFs) with the formula {[(Ln)3(mCB-L)4(NO3)(DMF)n]·Solv}, where Ln = Dy or Tb, characterized by dc and ac susceptibility, X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and heat capacity measurements. The MOF structure is formed by polymeric 1D chains of Ln ions with three different coordination environments (Ln1, Ln2, Ln3) running along the b-axis, linked by carborane-based linkers thus to provide a 3D structure. Static magnetic measurements reveal that these MOFs behave at low temperature as a system of S* = 1/2 Ising spins, weakly interacting ferromagnetically along the 1D polymeric chain (J*/kB = +0.45 K (+0.5 K) interaction constant estimated for Dy-MOF (Tb-MOF)) and coupled to Ln ions in adjacent chains through dipolar antiferromagnetic interactions. The Dy MOF exhibits slow relaxation of magnetization through a thermally activated process, transitioning to quantum tunneling of the magnetization at low temperatures, while both compounds exhibit field-induced relaxation through a very slow, direct process. The maximum magnetic entropy changes (-ΔSmaxm) for an applied magnetic field change of 2-0 T are 5.71 J kg-1 K-1 and 4.78 J kg-1 K-1, for Dy and Tb MOFs, respectively, while the magnetocaloric effect (MCE) peak for both occurs at T ∼ 1.6 K, approximately double that for the Gd counterpart.

Physicochemical characterization of B-hydroxyphenyl phosphine borane derivatives and their evaluation as nuclear estrogen receptor ligands

Increasing the structural options in medicinal chemistry is a promising approach to develop new drug candidates. In this research, we designed and synthesized a series of B-hydroxyphenyl phosphine borane derivatives and investigated their structure-property and structure-activity relationships. The synthesized B-phenylphosphine borane derivatives exhibited sufficient stability in aqueous media, weaker hydrophobicity than the corresponding alkanes and silanes, and sufficient affinity for lipid membranes to enable permeability. Several B-hydroxyphenyl phosphine borane derivatives exhibited significant estrogen receptor (ER) agonistic activity with superior ligand-lipophilicity efficiency (LLE). The phosphine borane framework appears to be a promising option for structural development in drug discovery studies.

In Vivo Application of Carboranes for Boron Neutron Capture Therapy (BNCT): Structure, Formulation and Analytical Methods for Detection

Carboranes have emerged as one of the most promising boron agents in boron neutron capture therapy (BNCT). In this context, in vivo studies are particularly relevant, since they provide qualitative and quantitative information about the biodistribution of these molecules, which is of the utmost importance to determine the efficacy of BNCT, defining their localization and (bio)accumulation, as well as their pharmacokinetics and pharmacodynamics. First, we gathered a detailed list of the carboranes used for in vivo studies, considering the synthesis of carborane derivatives or the use of delivery system such as liposomes, micelles and nanoparticles. Then, the formulation employed and the cancer model used in each of these studies were identified. Finally, we examined the analytical aspects concerning carborane detection, identifying the main methodologies applied in the literature for ex vivo and in vivo analysis. The present work aims to identify the current strengths and weakness of the use of carboranes in BNCT, establishing the bottlenecks and the best strategies for future applications.

Selective heterogeneous capture and release of actinides using carborane-functionalized electrodes

We report the heterogenization of molecular, electrochemically switchable ortho-substituted carboranes (^POCb, ^POCb-Pyr) for selective metal capture. Films of ^POCb and ^POCb-Pyr on glassy carbon and carbon fiber (CF) electrodes demonstrated heterogeneous electrochemical behaviour that was enhanced by the inclusion of single-walled carbon nanotubes (CNTs). Galvanostatically charged CF|CNT|^POCb and CF|CNT|^POCb-Pyr electrodes selectively captured and released actinides (Th⁴⁺, UO₂²⁺) from mixed solutions containing alkali (Cs⁺), lanthanide (Nd³⁺, Sm³⁺) and actinide (Th⁴⁺, UO₂²⁺) metal ions.

A luminescent Cu4 cluster film grown by electrospray deposition: a nitroaromatic vapour sensor

We present the fabrication and use of a film of a carborane-thiol-protected tetranuclear copper cluster with characteristic orange luminescence using ambient electrospray deposition (ESD). Charged microdroplets of the clusters produced by an electrospray tip deposit the clusters at an air-water interface to form a film. Different microscopic and spectroscopic techniques characterized the porous surface structure of the film. Visible and rapid quenching of the emission of the film upon exposure to 2-nitrotoluene (2-NT) vapours under ambient conditions was observed. Density functional theory (DFT) calculations established the favourable binding sites of 2-NT with the cluster. Desorption of 2-NT upon heating recovered the original luminescence, demonstrating the reusability of the sensor. Stable emission upon exposure to different organic solvents and its quenching upon exposure to 2,4-dinitrotoluene and picric acid showed selectivity of the film to nitroaromatic species.

Design, synthesis and structure-activity relationship of 4-(1,1,1,3,3,3-hexafluoro-2-hydroxyisoprop-2-yl)phenylsilane derivatives as liver X receptor agonists

Liver X receptor (LXR) α and LXRβ are nuclear receptors playing key roles in lipid metabolism, and LXR ligands are attractive drug candidates for metabolic disorders. Here we report the structural development of 4-(1,1,1,3,3,3-hexafluoro-2-hydroxyprop-2-yl)phenylsilane derivatives as LXR agonists bearing silyl functionalities as the hydrophobic pharmacophore, based on the structure of the known sulfonamide LXR agonist T0901317. Most of the synthesized compounds exhibit agonistic activity toward LXRs, but the LXR subtype-selectivity differs depending upon the substituents on the silicon atom. Among them, tri(n-propyl) derivative 12 shows potent LXR-agonistic activity with moderate α subtype-selectivity, while dimethylphenylsilyl derivative 19 shows modest β-selectivity. These results indicate that silanes can serve as an alternative to the sulfonamide moiety of LXR agonists, and are promising structural options for the development of novel subtype-selective LXR agonists.

Carboranes in drug discovery, chemical biology and molecular imaging

There exists a paucity of structural innovation and limited molecular diversity associated with molecular frameworks in drug discovery and biomolecular imaging/chemical probe design. The discovery and exploitation of new molecular entities for medical and biological applications will necessarily involve voyaging into previously unexplored regions of chemical space. Boron clusters, notably the carboranes, offer an alternative to conventional (poly)cyclic organic frameworks that may address some of the limitations associated with the use of novel molecular frameworks in chemical biology or medicine. The high thermal stability, unique 3D structure and aromaticity, kinetic inertness to metabolism and ability to engage in unusual types of intermolecular interactions, such as dihydrogen bonds, with biological receptors make carboranes exquisite frameworks in the design of probes for chemical biology, novel drug candidates and biomolecular imaging agents. This Review highlights the key developments of carborane derivatives made over the last decade as new design tools in medicinal chemistry and chemical biology, showcasing the versatility of this unique family of boron compounds.

Rational Design of Silicon-Based Zinc Ionophores

Ionophores transport ions across biological membranes and have wide-ranging applications, but a platform for their rapid development does not exist. We report a platform for developing ionophores from metal-ion chelators, which are readily available with wide-ranging affinities and specificities, and structural data that can aid rational design. Specifically, we fine-tuned the binding affinity and lipophilicity of a Zn^II -chelating ligand by introducing silyl groups proximal to the Zn^II -binding pocket, which generated ionophores that performed better than most of the currently known Zn^II ionophores. Furthermore, these silicon-based ionophores were specific for Zn^II over other metals and exhibited better antibacterial activity and less toxicity to mammalian cells than several known Zn^II ionophores, including pyrithione. These studies establish rational design principles for the rapid development of potent and specific ionophores and a new class of antibacterial agents.

Water-Stable Carborane-Based Eu3+/Tb3+ Metal-Organic Frameworks for Tunable Time-Dependent Emission Color and Their Application in Anticounterfeiting Bar-Coding

Luminescent lanthanide metal-organic frameworks (Ln-MOFs) have been shown to exhibit relevant optical properties of interest for practical applications, though their implementation still remains a challenge. To be suitable for practical applications, Ln-MOFs must be not only water stable but also printable, easy to prepare, and produced in high yields. Herein, we design and synthesize a series of m CB-Eu _y Tb _1-y (y = 0-1) MOFs using a highly hydrophobic ligand mCBL1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. The new materials are stable in water and at high temperature. Tunable emission from green to red, energy transfer (ET) from Tb³⁺ to Eu³⁺, and time-dependent emission of the series of mixed-metal m CB-Eu _y Tb _1-y MOFs are reported. An outstanding increase in the quantum yield (QY) of 239% of mCB-Eu (20.5%) in the mixed mCB-Eu_0.1Tb_0.9 (69.2%) is achieved, along with an increased and tunable lifetime luminescence (from about 0.5 to 10 000 μs), all of these promoted by a highly effective ET process. The observed time-dependent emission (and color), in addition to the high QY, provides a simple method for designing high-security anticounterfeiting materials. We report a convenient method to prepare mixed-metal Eu/Tb coordination polymers (CPs) that are printable from water inks for potential applications, among which anticounterfeiting and bar-coding have been selected as a proof-of-concept.

Structural development of silicon-containing retinoids: structure-activity relationship study of the hydrophobic pharmacophore of retinobenzoic acids using silyl functionalities

We designed and synthesized a series of retinobenzoic acids bearing various silyl functionalities in order to explore in detail the structure-activity relationship (SAR) at the hydrophobic moiety of retinoids. Among the synthesized compounds, 24c bearing a t -butyldimethylsilyl (TBS) group at the hydrophobic site exhibited potent retinoid activity comparable to that of the lead compound Am555S ( 4 ). Compound 24c exhibited transcription-promoting activity towards all three subtypes of retinoic acid receptor (RAR), but showed the highest activity towards RARγ, in contrast to the high RARα-selectivity of Am80 ( 3 ) and Am555S ( 4 ). The SARs presented here should be helpful in the development of subtype-selective retinoids, and in particular 24c might be a promising lead compound for new RARγ ligands.

Rational design of carborane-based Cu2-paddle wheel coordination polymers for increased hydrolytic stability

A new unsymmetric carborane-based dicarboxylic linker provided a 1D Cu₂-paddle wheel coordination polymer (2) with much higher hydrolytic stability than the corresponding 2D Cu₂-paddle wheel polymer (1), obtained from a related more symmetrical carborane-based linker. Both 1 and 2 were used as efficient heterogeneous catalysts for a model aza-Michael reaction but only 2 can be reused several times without significant degradation in catalytic activity.

Ruthenacarborane-Phenanthroline Derivatives as Potential Metallodrugs

Ruthenium-based complexes have received much interest as potential metallodrugs. In this work, four Ru^II complexes bearing a dicarbollide moiety, a carbonyl ligand, and a phenanthroline-based ligand were synthesized and characterized, including single crystal diffraction analysis of compounds 2, 4, and 5 and an observed side product SP1. Complexes 2-5 are air and moisture stable under ambient conditions. They show excellent solubility in organic solvents, but low solubility in water.

New keys for old locks: carborane-containing drugs as platforms for mechanism-based therapies

Icosahedral carboranes in medicine are still an emerging class of compounds with potential beneficial applications in drug design. These highly hydrophobic clusters are potential "new keys for old locks" which open up an exciting field of research for well-known, but challenging important therapeutic substrates, as demonstrated by the numerous examples discussed in this review.